Minecraft Bedrock Air World?

This Wiki page has some information about customizing super-flat worlds on Bedrock edition. It appears there's no GUI to do it like in Java Edition Minecraft:

Bedrock Edition has no interface for customizing flat worlds; it always uses the default configuration (one layer of bedrock, two layers of dirt, and a layer of grass blocks) even when the Seed Picker is used. However, custom flat worlds are supported internally, and can be used by modifying FlatWorldLayers in the world's level.dat file using an external editor.

The editor used to edit level.dat appears to be Universal Minecraft Editor, thanks MrParrot